June 22, 2007—Under the peaceful Mediterranean island of Santorini, Greece, simmers a mostly submerged volcano that was once responsible for the second biggest eruption in human history.

In 1630 B.C., the Santorini volcano exploded, completely burying its thriving civilization of Akrotiri. Some historians believe the event also ushered in the demise of the entire Minoan culture, which flourished on nearby Crete from roughly 2700 to 1450 B.C.

In 2006 National Geographic Explorer-in-Residence Bob Ballard—who also discovered the Titanic—set out to investigate the volcano's violent past. (National Geographic News is part of the National Geographic Society.)

With undersea rovers, Ballard and his team measured the volcano's layers—and concluded the eruption 3,500 years ago was bigger than previously thought. The event likely spewed 14 cubic miles (58 cubic kilometers) of magma onto the nearby islands. Massive tsunamis crashed onto shore and ash blackened skies for miles.

Follow the explorers underwater as they trace the volcano's history, and learn why experts believe it could have a turbulent future.